1. Field of the Invention
This invention relates to a method of improving the texture of aluminum metallization for tungsten etch back processing.
2. Brief Description of the Prior Art
In the fabrication of semiconductor devices, an often required step is that of chemical vapor deposited (CVD) tungsten plug processing wherein a titanium/titanium nitride stack is deposited prior to the CVD tungsten deposition. Etch back is then performed to remove the unwanted tungsten on the field oxide in order to reduce the lead resistance. Aluminum alloy films are then deposited for the leads. Typically, the tungsten etch back is stopped by the titanium nitride layer and a titanium nitride/aluminum/titanium nitride (TiN/Al/TiN) stack is deposited for the lead because better electromigration resistance is observed for this type of stack structure.
It has, however, been observed that there is texture or crystallographic orientation degradation of the TiN/Al/TiN stack when that stack is disposed over an underlying Ti/TiN stack which has gone through the tungsten etch back processing. It is desired that the (111) crystallographic orientation of the aluminum be, to the greatest possible extent, normal to the surface of the aluminum layer. For example, without etch back, the TiN/Al/TiN stack can have the texture of aluminum with (111) crystallographic orientation (Al(111)) texture of about 1.4 degrees for x-ray rocking curve full-width-at half-maximum (FEWM. However, after etch back, the FWHM dropped to about 3.5 degrees. This change is attributed to the damage to the TiN surface due to the tungsten etch back processing prior to the deposition of the aluminum.
Aluminum with (111) texture in a direction normal to the surface of the aluminum layer is most beneficial for electromigration improvements. The texture is controlled by the deposition conditions and, most profoundly, by the substrates (e.g. aluminum films can develop strong (111) texture when titanium is used beneath the aluminum). However, to prevent interaction between the aluminum and the titanium, it is necessary to form a layer of titanium nitride or other barrier material between the aluminum and the titanium. Also, the titanium nitride layer can prevent the reaction between tungsten hexafluoride (WF.sub.6) (generally used for tungsten plug filling) and titanium. Fortunately, titanium nitride has an atomic arrangement similar to that of aluminum (111) and titanium (0001). Therefore, by controlling the orientation of the titanium, the texture of the titanium can be transferred to the titanium nitride and then to the aluminum.
In accordance with standard fabrication techniques there is provided a substrate having a metal layer and an oxide layer thereon with the oxide layer extending over the metal layer and having a via extending therethrough to the metal layer, such as, for example, silicon dioxide on silicon. A layer of titanium is deposited over the oxide layer including the interior walls of the via and a titanium nitride layer is deposited over the entire layer of titanium including the interior walls of the via. Tungsten is then deposited over entire surface to cover the titanium nitride layer, fill the via. The tungsten layer is thereby separated from the oxide layer by the titanium and the layer of titanium nitride. The tungsten layer is then etched back using an appropriate etchant, preferably SF.sub.6 as the etchant, with the titanium nitride layer acting as an etch stop so that the tungsten remains only within the via. As a result of this etch back of tungsten step, a degree of contamination remains on the surface of the titanium nitride layer and exposed tungsten within the via in the form of elemental sulfur, fluorine and possibly other contaminants. Accordingly, when an aluminum interconnect layer is then deposited over the exposed titanium nitride and tungsten within the via, due to the contamination, very little of the aluminum will have the desired (111) crystallographic orientation normal to the surface of the aluminum. As described above, it is highly desirable to have as much of the aluminum as possible having the (111) crystallographic orientation normal to the surface of the aluminum. Accordingly, a solution to this problem is highly desirable.